This one is quite easy.
After you find the boat, Kenny tells you it needs fuel and a battery. You all decide to go to Crawford as it is the most likely place that will have them.
As you come out of the sewer into crawford Vernon will comment on the lack of guards. One guard is seen patrolling near by. When you approach to take him out you find out he is a walker. Then loads more appear.
You all run inside and you get the achievement.